1. Field of the Invention
The present invention relates to high energy, non-aqueous electrolyte based electrochemical energy storage devices such as high energy density batteries or high power electrochemical capacitors. More particularly, this invention relates to high energy, non-aqueous electrolytes based electrochemical energy storage devices containing an electrolyte solution including isocyanate additives, which afford improved capacity retention at wide temperature ranges and increased reliability for PC-based electrolytes.
2. Discussion of the Prior Art
High voltage and high energy density rechargeable batteries based on non-aqueous electrolyte solutions are widely used as electric sources for various types of consumer electronic appliances, such as camcorders, notebook computers, and cell phones, because of their high voltage and high energy density as well as their reliability such as storage characteristics. This type of battery conventionally employs the complexed oxides of lithium and a transition metal as a positive electrode, such as LiCoO2, LiNiO2, LiMn2O4, and variations of the previous oxides with different dopants and different stoichiometry, and additionally includes lithium metal, lithium alloys and/or carbonaceous materials as a negative electrode. Chosen over the lithium metal and lithium alloys are carbonaceous negative electrode materials, which are in general partially or fully graphitized and specially modified natural graphite. When a carbonaceous negative electrode is used, this battery is often referred to as a lithium-ion (Li-ion) battery, because no pure lithium metal is present in the negative electrode. During charge and discharge processes, the lithium ions are intercalated into and de-intercalated from the carbonaceous negative electrode, respectively. The advantage of using these negative electrodes is that problems associated with growth of lithium metal dendrites is avoided. Such dendrites are often observed in lithium or lithium alloy negative electrodes, and are known to cause short-circuiting of the cells.
Non-aqueous electrolytes used in the-state-of-the-art lithium-ion batteries contain a solvent system that, in general, includes a cyclic carbonate compound, such as ethylene carbonate (EC) and propylene carbonate (PC), as well as a linear carbonate, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC). The cyclic carbonates are chemically and physically stable and have high dielectric constant, which is necessary for their ability to dissolve salts. The linear carbonates are also chemically and physically stable and have low dielectric constant and low viscosity, which is necessary to increase the mobility of the lithium ions in the electrolytes. “PC-based electrolyte system” contains PC as one of the components and, when the only cyclic carbonate present is EC, the electrolyte system is considered “EC-based”.
PC is oxidatively more stable than EC and has lower melting point, therefore, the electrolytes composed of PC have wider liquid temperature ranges. However, PC is not a preferred solvent in rechargeable batteries, especially in Li-ion batteries when the anode is made of a graphite-based carbonaceous material. This is because PC molecules co-intercalate together with Li ions into carbonaceous anode materials and decompose between graphite layers or the surface of carbonaceous anode, which subsequently exfoliates carbonaceous anode and yields some gases inside the batteries. These problems not only shorten the life and performance of the batteries, but also raise safety concerns with such batteries because of a build-up of the internal pressure. However, PC is a preferred solvent in rechargeable Li-ion batteries when the anode is made of amorphous hard carbon. This is because PC cannot co-intercalate into the structure of this type of carbon and therefore it becomes possible to take the advantages of PC in rechargeable Li-ion batteries.
The solutes conventionally used in typical electrolytes are lithium salts such as lithium hexaflurophosphate (LiPF6), lithium imide (LiN(SO3CF3)2), lithium trifluoromethane sulfonate (LiCF3SO3), lithium hexafluoroarsenate (LiAsF6), and lithium tetrafluoroborate (LiBF4).
In terms of cost and performance, graphite is a preferred anode material for rechargeable Li-ion batteries. Therefore, it is desirable to use graphite or graphitizable carbonaceous materials as anode materials with a highly oxidatively stable solvent such as PC. To avoid the problems of PC with graphite as mentioned above, it is desirable to develop an electrolyte system that can form a protective layer on the surface of graphite so that Li ions can pass through the layer but PC cannot.
Furthermore, it is desirable to improve Li-ion batteries with a graphite or graphitizable carbonaceous anode even for the electrolytes containing no PC solvent. This is because the protection layer formed on graphite surfaces in electrolytes composed of EC and linear carbonate solvents is resistive at temperatures below −20° C. (see, for example, Plichta et al., “Low Temperature Electrolyte for Lithium and Lithiun-Ion Batteries,” Proc. 38th Power Sources Conference, p. 444, Cherry Hill, N.J., 8-11 Jun. 1998, herein incorporated by reference in its entirety). At temperatures above 50° C., the protection film loses its ability to protect the graphite electrode. As a result, the battery cannot retain its charge capacity through storage and cycling at elevated temperatures. Therefore, it is also desirable to improve the Li-ion battery with graphite anode and electrolytes containing no PC.